Dry roast the tomatillos, jalapeños, serranos, white onion, and garlic cloves in a skillet over medium-high heat until completely cooked through and charred. Some ingredients will cook faster than others, so remove them as they’re ready to avoid burning.
Once cooked, add them all to a blender along with 1/4 cup of water and blend until your desired consistency is reached. Keep it slightly chunky for dipping or blend smooth for a pourable salsa.
Thin out the salsa with extra water if needed, then season with salt and a pinch of cumin.
Stir in chopped onion, cilantro, and avocado if using. Serve and enjoy immediately or chill for later.